Method for the production of a piston pin

ABSTRACT

For the production of piston pins having a hard peripheral area, a relatively inexpensive chill casting method is used. A chill form having a mold cavity for a piston pin is embedded in a casting mold, cast iron is poured into the mold cavity of the chill form, and then the peripheral area of the piston pin that results from this process is cooled by means of the chill form, is white-solidified, and thereby forms a hard wear-resistant surface.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and a device for the production of a piston pin having a hard peripheral area.

2. The Prior Art

It is known from the state of the art to manufacture piston pins having a peripheral area that has sufficiently great hardness that the use of bearing shells may be eliminated. Offenlegungsschrift DE 197 04 224 describes a cylindrical piston pin whose mantle surface has a layer of amorphous carbon. This piston pin is directly mounted in the eye of the connecting rod and in the pin boss of the piston, avoiding slide bearing shells.

From U.S. Pat. No. 1,742,015, it is known to encase a steel tube in order to produce a piston pin, after which the casting material is hardened by means of rapid cooling, using a cooling mantle that is supplied with a coolant.

The methods for the production of piston pins having a hardened outside surface known from the state of the art have the disadvantage that they are very complicated and costly.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method for producing a piston pin that has a hard surface, which can be carried out simply and therefore inexpensively.

These and other problems are solved by a chill casting method and device according to the invention. In one aspect, a chill form having a mold cavity for at least one piston pin is embedded in a casting mold. Cast iron is poured into the mold cavity of the chill form. The peripheral area of the piston pin or pins that results from this process is chilled and forms a hard, ledeburite structure.

In another aspect, a device is provided for implementing the method according to the invention in which the casting mold is a sand mold. Practical further developments are discussed below.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and features of the present invention will become apparent from the following detailed description considered in connection with the accompanying drawings. It should be understood, however., that the drawings are designed for the purpose of illustration only and not as a definition of the limits of the invention.

In the drawings,

FIG. 1 is a cross-sectional view through a sand mold having a chill form, filled with cast iron that has already solidified, for the production of a piston pin;

FIG. 2 is a side view of an embodiment of a chill form made up of two half-shells, for the simultaneous production of two piston pins;

FIG. 3 is a cross-sectional view of the chill form along the line III-III in FIG. 2;

FIG. 3A is a perspective view of another embodiment of a chill form; and

FIG. 4 are micrographs of the piston pin, to show the structure of the cast material in the peripheral area and the inner region of the pin.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Turning now in detail to the drawings, FIG. 1 shows part of a sand mold 1 for casting a cylinder-shaped piston pin, into which a chill form 3 in the form of a cylindrical tube is embedded, which has a mold cavity for a piston pin. The casting method that can be carried out with this mold is called the chill casting method.

Sand mold 1 is shown with a completed, cast piston pin 2 after solidification, whereby the peripheral area 4 of piston pin 2 solidifies in a white color, because of the rapid cooling effected by chill form 3, and its inner region 5, which cools more slowly than peripheral area 4, assumes a dark color. Subsequent to the casting process, after the casting has been removed from chill form 3, its mantle surface is finished in conventional manner, i.e. it is ground and polished.

A ledeburite structure having a marked carbide texture forms in peripheral area 4, within the scope of the casting process, because of the high cooling speed, giving this region great hardness and wear resistance. Inner region 5 of piston pin 2 does not assume the hardness achieved by peripheral area 4. Because of the lower cooling speed, a pearlite structure is formed at inner region 5, in which the carbon is at least partially present as graphite. This graphite gives the inner region its dark color.

In this connection, cast iron with spheroidal graphite is used, i.e. an iron/carbon casting material whose carbon content, which is present as graphite, is present almost entirely in essentially spheroid, i.e. in spherolitic form.

Such cast parts have a hard, wear-resistant surface and improved ductility in the core and, therefore, can absorb shock and impact stress to a higher degree.

Examples of such chemical compositions of the cast iron are:

Material Example a):

-   3.4 to 3.9 wt.-% carbon (C) -   1.7 to 2.6 wt.-% silicon (Si) -   0.2 to 0.4 wt.-% manganese (Mn) -   0 to 0.1 wt.-% phosphorus (P) -   0.03 to 0.08 wt.-% magnesium (Mg) -   0 to 0.015 wt.-% sulfur (S);     Material Example b): -   3.0 to 3.8 wt.-% carbon (C) -   1.2 to 1.6 wt.-% silicon (Si) -   0.2 to 1.0 wt.-% manganese (Mn) -   0 to 0.1 wt.-% phosphorus (P) -   0.2 to 1.0 wt.-% copper (Cu) -   0.2 to 0.4 wt.-% molybdenum (Mo) -   0 to 0.02 wt.-% sulfur (S).

FIG. 2 shows another embodiment of a chill form 6, in a side view; it is made up of two half-shells 7 and 8 that are “E”-shaped in cross-section, which form mold cavities 9, 10 for two piston pins to be produced by them, when their faces are laid onto one another. Chill form 6 is made of a material with high thermal conductivity, for example gray cast iron.

FIG. 3 shows chill form 6 in cross-section along the line III-III in FIG. 2.

FIG. 3A shows another embodiment of a chill form 6′ made up of two “E”-shaped half shells 7′ and 8′, in section. The hollow cylinder-shaped chill form 6′ has webs at its two ends 14, 16 and in its center region 15. This configuration creates a piston pin with a hard, wear-resistant peripheral area in the center, forming the contact region piston pin/connecting rod, and at its two ends, forming the contact region piston pin/pin boss.

FIG. 4 shows micrographs of piston pin 2, whereby micrograph 11 shows the entire cross-section of the piston pin, micrograph 12 shows the structure of peripheral area 4 of piston pin 2, and micrograph 13 shows the structure of inner region 5 of piston pin 2, in enlarged form. Micrograph 12 of outer, white region 4 of piston pin 2 shows the carbide texture of the ledeburite metal structure, which is formed when the peripheral area of the casting is quickly chilled.

Micrograph 13 clearly shows the graphite which is formed in a globular or spheroid shape as a result of the slow cooling speed.

Although only a few embodiments of the present invention have been shown and described, it is to be understood that many changes and modifications may be made thereunto without departing from the spirit and scope of the invention as defined in the appended claims. 

1. A method for producing a piston pin having a hard peripheral area comprising the following steps: (a) embedding a chill from having a mold cavity for at least one piston pin in a casting mold; (b) pouring cast iron into the mold cavity of the chill form to form at least one piston pin; and (c) conducting accelerated cooling of at least a portion of the peripheral area by means of the chill form to form a hard ledeburite structure.
 2. The method according to claim 1 wherein the cast iron comprises nodular cast iron with graphite mainly in spherolitic form.
 3. The method according to claim 1 wherein the casting mold is a sand mold.
 4. The method according to claim 1 wherein the chill form comprises a cylindrical tube.
 5. The method according to claim 1 wherein the chill form comprises a hollow cylinder having first and second ends and a center region between the ends, the ends and the center region having webs for concentrating the chill effect in the peripheral area of the center region and the ends, the center region forming a contact region piston pin to connecting rod, and each end forming a respective contact region piston pin to pin boss.
 6. The method according to claim 1 wherein the chill form has a shape of an “8” and comprises two “E”-shaped halves in cross-section.
 7. A device for producing a piston pin having a hard peripheral area comprising: (a) a sand mold; and (b) a chill form having a mold cavity embedded in said sand mold.
 8. The device according to claim 7 wherein said chill form comprises a cylindrical tube.
 9. The device according to claim 7 wherein said chill form comprises a hollow cylinder having first and second ends and a center region between said ends, said ends and said center region having webs for concentrating the chill effect in the peripheral area of the center region to form a contact region piston pin to connecting rod and the ends to form at each end a respective contact region piston pin to pin boss.
 10. The device according to claim 7 wherein said chill form has a shape of an “8” and comprises two “E”-shaped halves in cross-section. 